The present invention relates to a method and apparatus for recovery of uranium from carbonate solutions, such as leach liquors resulting from leaching of uranium deposits with a solution of sodium or ammonium carbonate or bicarbonate, or even carbon dioxide solutions to form carbonate solutions having a pH in the range of about 4.2 to about 9.8. A further embodiment of the present invention is directed toward restoring the liquid remaining after the uranium has been removed to a purified condition suitable, for instance, for return to the groundwater.
Much of the known uranium now existing in the world is in ores with uranium contents below 0.2% U.sub.3 O.sub.8. One typical method to remove uranium from such low grade ores is to leach the ore, either above the ground or under the ground (in situ leaching), to produce a fairly low grade uranium leach liquor, and then to recover the uranium from the leach liquor. The end product is typically referred to as uranium yellow cake, which is usually sent to a refinery where it is further purified into a form suitable for enrichment and fuel fabrication.
The waste liquor following uranium recovery must be treated, particularly during in situ leaching operations. It must be restored to the original groundwater conditions prior to disposal. Recovery of uranium during such a restoration process is highly desirable.
Reverse osmosis, with or without a preceeding lime softening step, is one known technique for restoring such solutions. Residual uranium is not always recoverable through such a technique, because reverse osmosis is not selective; it also removes a large number of other dissolved solids from which the uranium must later be separated. Furthermore, the reverse osmosis membranes scale easily, and pretreatment of reverse osmosis influent feed with acid and polyphosphates is not always an efficient and inexpensive way to prevent scaling.
One of the most inexpensive acid leaching systems utilizes sulfuric acid, hydrochloric and nitric acid are effective though sometimes considered too expensive. Sulfuric acid leaching provides a leach liquor which contains many cations, some of which are divalent or trivalent. Some of these cations are Fe.sup.+3, Fe.sup.+2, Al.sup.+3, Mg.sup.+2, Ca.sup.+2, Mn.sup.+2, Zn.sup.+2, Na.sup.+, and K.sup.+.
Typical prior art methods to recover uranium from such leach liquors include precipitation and anion exchange. Some of these methods are disclosed in two books by Robert Kunin, one of the inventors of the present invention: Elements of Ion Exchange (1971) at 102-109, and Ion Exchange Resins (1972) at pages 190-197.
These latter two prior art methods also present problems. For instance, direct precipitation of uranium from acid leach liquors with such compounds as ammonia is not preferred because the precipitate does not contain a significantly greater percentage of uranium than the original ore. Also, anion exchange processes adsorb, in addition to uranium anions, HSO.sub.4.sup.-, SO.sub.4.sup.-2, and a number of other anion sulfate complexes containing titanium, zirconium, vanadium, molybdenum, iron and thorium. Vanadium, iron, and molybdenum are often adsorbed in highly undesirable quantities. Prior art techniques to accommodate this unwanted adsorption of vanadium and molybdenum include selective elution, as discussed in U.S. Pat. No. 2,864,667 (Bailes et al.), adjusting the pH of the leach solution, as discussed in U.S. Pat. No. 2,841,468 (Wilson), and contacting a reduced pH eluate with activated carbon, as discussed in U.S. Pat. No. 4,092,399 (Narayan et al.). Such known techniques do not alter the fact that anion exchange resins often adsorb molybdenum and vanadium in highly undesirable quantities.
While it has been hypothesized that highly selective cation exchange resins might be able to work on uranium sulfate leach liquors (see Kunin, Elements of Ion Exchange 104 (1971)), the process generally used is an anion exchange process, as discussed in Merrit, The Extractive Metallurgy of Uranium 137-63 (1971). Cation exchange processes on the products of sulfuric, hydrochloric, or nitric acid leaching adsorb, along with the uranium, even more unwanted ions than anion exchange processes, such as calcium, magnesium, iron, aluminum and so forth. Liquid cation exchangers such as perfluorooctanoic acid and di-2-ethylhexylphosphoric acid (D2EHPA), while used in some uranium plants on acid leach liquors, typically have high solubility and a tendency to form emulsions. It is not believed that liquid cation exchangers have previously been used on carbonate leach liquors.
It should be noted at this point that, throughout the application herein, the term "adsorption" is used to refer both to processes or ion exchange which occur at surface of ion exchange resins and to those processes which occur throughout the entire resin structure. By using the term adsorption in this application, including the claims herein, applicants do not intend to limit the claims to processes or apparatus involving surface actions on ion exchange resins.
While the above-noted problems occur when prior art leaching methods with sulfuric, hydrochloric, or nitric acid are used, leaching with hydrochloric or nitric acid has particular drawbacks. Cation exchange techniques on chloride solutions using Amberlite IRC-50 are discussed in the U.S. Atomic Energy Commission Report RMO-2502 (now declassified) for July 1, 1951 through Aug. 1, 1951, and disadvantages with increased chloride concentration and the addition of calcium were reported. Furthermore, anion exchange does not efficiently remove uranium from hydrochloric or nitric acid leach liquors. In hydrochloric acid solutions the uranyl cation forms anionic complexes but these are much weaker complexes than those formed with the iron, zinc, and other cations that are present in the solution. Therefore, anion exchange will not be sufficiently selective on the hydrochloric leach liquor to efficiently remove uranium. Also, anion exchange will not remove uranium from nitric acid leach liquors because the uranyl cation, UO.sub.2.sup.+2, does not form anionic complexes with nitrates.
Sulfuric, hydrochloric or nitric acid leaching agents cannot be used economically with ores having a high limestone content. In such cases, leaching of the uranium can be achieved on the alkaline side with a solution of sodium or ammonium carbonate and/or bicarbonate, or even with carbon dioxide solutions. It is the product of that leaching process that the present invention is directed toward treating. For the purposes of this description and the claims herein, the phrase "carbonate solution" is used in a broad sense to include carbonate leach liquors having a pH of about 4.2 to about 9.8, which are produced by sodium or ammonium carbonate and/or bicarbonate or by a carbon dioxide solution, reacting with the alkaline ores being leached.
Such a carbonate solution probably contains complex carbonate ions, and it is believed that the ionic species are governed by the following equilibrium reaction although the underlying processes are not fully understood: EQU UO.sub.2.sup.+2 +(CO.sub.3.sup.-2).sub.n .revreaction.[UO.sub.2 (CO.sub.3).sub.n ].sup.2-2n
Probably the most common anoinic complex present is EQU [UO.sub.2 (CO.sub.3).sub.2 ].sup.-2
so that the uranyl carbonate anion complex is probably in equilibrium with the uranyl cation as follows: EQU UO.sub.2.sup.+2 +2(CO.sub.3.sup.-2).revreaction.[UO.sub.2 (CO.sub.3).sub.2 ].sup.-2
According to prior art methods, such as disclosed in Kunin, Ion Exchange Resins (1972) at pages 195-97, the uranium can be recovered from the carbonate solution in several ways. First, alkali could be added to precipitate Na.sub.2 U.sub.2 O.sub.7. The filtrate from this precipitation would be recarbonated with carbon dioxide for reuse in leaching more ore. Second, the carbonate solution could be acidified with hydrochloric acid and boiled to remove carbon dioxide, with uranium precipitated as the hydroxide by adjusting the pH to neutrality. Last, it appears that anion exchange could operate on the anionic uranium complex to recover the uranium. Ion exchange recovery is particularly useful when the digested pulp presents difficult filtration problems. Since many carbonate ores contain bentonite clay, filtration is often difficult, leaving ion exchange on the unclarified pulp as the most attractive prior art method in such a situation.
Known prior art methods of recovering uranium from carbonate leach liquors utilized anion exchange, even though cation exchange was considered a possibility for treating chloride leach liquors, and a highly selective cation exchange resin might remove uranium from a sulfate leach liquor. Such anion exchange processes for removal of uranium from carbonate leach liquors are discussed in U.S. Pat. No. 4,155,982 (Hunkin et al.), U.S. Pat. No. 2,982,605 (Mouret et al.), U.S. Pat. No. 2,811,412 (Poirier), U.S. Pat. No. 2,780,514 (Lutz), and Merrit, The Extractive Metallurgy of Uranium 151-56 and 161-63 (1971). However, molybdenum and vanadium, often present in carbonate leach solutions, are also adsorbed in undesirable quantities by the anion exchange resin, as discussed above. Prior to the present invention, it was not considered feasible to use a cation exchange process on carbonate leach solutions containing uranium, because of the relatively high level of cations that would be separated from solution along with the uranium just as when cation exchange processes are used on sulfuric, hydrochloric or nitric acid leach solutions.